Research On Passive Harmonic Mode-locking Fiber Laser Based On Topological Insulator

Harmonic mode-locking fiber lasers become a hot subject due to their potential applications in many fields,such as optical communication,fiber sensors,biomedical research,laser radar system and nonlinear optics.Make use of saturable absorber(SA)to generate ultra-short pulse,the structure compact,pulse width narrower and operate stability.In the recent years,topological insulators(TIs)gain huge attention in physics,chemistry and material fields due to their huge nonlinear refractive index,high modulation depth,and low saturable absorption threshold.This paper studies on the usage of topological insulator saturable absorption and nonlinear effect to achieve harmonic mode-locked pulse.In the experiment,we obtained the different spectrum forms of pulse by adjusting the dispersion,loss,polarization parameters in the optical fiber.The multi-wavelength spectrum form is produced with the highly nonlinear fiber.Therefore,the solitons mode-locking pulse is obtained by the balance of dispersion and nonlinear effect.In addition,to change the pump power and the polarization state of the resonant cavity,the high order harmonic pulse can appear in fiber cavity,in order to solve the repetition frequency limitation of the cavity length,and the power higher,quality better of mode-locked pulse.The main contents are as follows:First of all,analyze the advantages of fiber laser and the important optical devices in resonant cavity,the basic theory of active and passive mode-locking.Whereby,it also explains the mechanics of different passive mode locking methods.We focused on the nonlinear effects,dispersion and other parameters in fiber,and present the impact of these factors on the pulse in detail.The dispersion can broaden the pulse and self-phase modulation can narrowed it.When these two balanced,the solitons pulse is formed.Secondly,it is analysis the formation mechanism of the multi-wavelength mode-locked pulse.It is originated from the filtering effect produced by saturation absorption of TIs,nonlinear birefringence,nonlinear effect and polarization dependent loss of polarization controller.In the experiment,we use two TIs film to achieve the dual-wavelength and triple-wavelength pulses respectively.In near zero dispersion region,it can also operate at fundamental frequency and higher harmonic solitons mode-locking by using the mode-locker unit.Then explains the formation mechanism and evolution law of the mode-locking pulse.Furthermore,we also analyzed the influence of the solitons pulse shape and pulse width by nonlinear effect,cavity dispersion.Finally,it is confirmed that TIs can achieve mode-locking pulse through the experiment,and it can obtain different spectral state of mode-locked pulses by adjusting the parameters of fiber resonator.At the same time,the laser not only work at the fundamental frequency,but also at the higher harmonic order.The harmonic order continuously increases by increasing the pump power.By adjusting the polarization state,it will change the loss of the resonant cavity,thus the pulses pass by the different optical path in resonant cavity,therefore can present a different harmonic mode locking order.We can also obtain different harmonic order of the mode-locking by adjusting the polarization controller in the experiment.